Abstract:

Shale samples from the lower Cambrian Yanjiahe-Shuijingtuo formations in Yichang, western Hubei were collected by high-resolution sampling method to investigate the vertical variation of biogenic silica and its significance for shale gas enrichment, by combining quantitative analyses of total organic carbon (TOC), major elements and helium porosity with qualitative microscopic observation using transmission and cathodoluminescence microscope. According to the results, the excess silicon content is low in the organic-poor Lower Cambrian Yanjiahe-upper Shuijingtuo formations containing no visible trace of bioclastic debries and very little or barely no authigenic quartz. While the organic-rich layer of the Shuijingtuo formation has larger quantity of biogenic silicon and is divided into members 1 and 2 as the silicon sources are obviously different. The bottom part (member 1) has high excess silicon content ranging between 4.44%-24.97% (average 16.76%), with significant fluctuation under the influence of calcareous intercalation; while biogenic silicon, mainly siliceous sponge spicules and a few flocculent siliceous aggregates, accounts for a high proportion with little influence from terrigenous clasts; and organic matter could be seen in the spicule cavity and interparticle pores of siliceous aggregates. In contrast, the middle-upper part (member 2), influenced by both biogenic silicon and terrigenous clasts, has significantly lower excess silicon content ranging between 7.13%-20.47% (average 13.66%), with radiolarian, fusiform siliceous aggregates and sponge spicules as the main bioclastic types and mixed with organic matter. TOC is positively correlated with excess silicon in the organic-rich strata, and large numbers of fine granular chalcedony are observed under microscope in lumps of organic matter developed in an algal cyst-like structure, which reflects that the development of high hardness crystalline biogenic silicon, transformed from opal in the organic-rich layer is conducive to the enrichment of organic matter. Compared to member 2, the TOC content in member 1 is significantly higher and porosity relatively low, which may correspond to the difference in biogenic silicon contents in the two members. The close symbiosis between biogenic silicon and terrigenous clasts may play a protective and supporting role in the development and preservation of pores. The quartz origin may be the main factor influencing the gas-bearing property of shales from the organic-rich layers of the Cambrian Shuijingtuo formation in Yichang area.

Key words: western Hubei, Cambrian, shale gas, Shuijingtuo Formation, biogenic quartz, pore